1. Field of the Invention
The present invention relates to a light modulation device, and more particularly to a light modulation device which is capable of modulating light in the infrared wavelength region to the visible ray region and performing exposure and display using light in the foregoing region.
2. Description of the Related Art
Hitherto, devices for modulating light, exposing a photosensitive material to light or performing display by using the modulate include an electro-optical crystal, such as LN (lithium niobate: LiNbO3), KDP (potassium phosphate: KH2PO4) or ADP (ammonium phosphate: NH4H2PO4), using the Pockels effect or electro-optical crystal, such as PLZT, mainly using the Kerr effect.
The foregoing liquid crystal devices, which can be operated at low voltage, however, suffer from unsatisfactorily low response speed and excessive dependency on the environment including temperature. TN (Twist Nematic) type liquid crystal and birefringent liquid crystal, which are main types of the liquid crystal, must be used with a polarizing plate. Thus, there arises a problem of light absorption owning to the polarizing plate.
As a new light modulation device, which is capable of overcoming the foregoing problems, a technique has been disclosed in Japanese Patent Laid-Open No. 9-179082. As shown in FIG. 16, dielectric substances 113, such as PZT or PLZT, are, in a space between glass substrates 111 and 112, held between transparent electrodes 114 and 15. Then, voltage is applied so that light modulation is performed. The principle which is used to perform the light modulation is as follows: when the space between the two transparent electrodes is not applied with voltage, penetration of light is permitted. When voltage is applied to the space, electrons are charged in the cathode side of the transparent electrode. Thus, the density of electrons is raised so that light is reflected by the transparent electrode adjacent to the cathode. The foregoing method enables light modulation to be performed without the polarizing plate.
In the foregoing light modulation process, the density of electrons required to perform the light modulation is raised as the wavelength of light is shortened. Therefore, enlargement of the number of electrons, which must be charged, is required to induce raising of the density of electrons in the light modulation device. To operate the device, the electrostatic capacity between the electrodes must be enlarged. Therefore, the foregoing dielectric substance must be a material, for example, PZT or PLZT which is a ceramic-type high dielectric-constant substance, having a significantly high dielectric constant. The foregoing materials having the high dielectric constant, however, cannot easily be formed into a thin film having a stable dielectric constant. Therefore, there arises a problem in that the density of electrons around the transparent electrode cannot stably be controlled.
In recent years, light modulation devices using the field-optical effect of the semiconductor or control of the density of carriers in a p-n junction structure have been developed. All of the developed devices are arranged to perform light modulation in the vicinity of the wavelength near an end of light absorption by using band gap energy. There has not been developed a device of a type for performing the light modulation in the visible ray region or changing the reflectance of light in the foregoing region.
An electric optical crystal is able to perform a high-speed response and is free from excessive dependency on the environment. However, an electric optical crystal of a type using the Pockels effect requires very high operating voltage. An electric optical crystal of a type using the Kerr effect can be operated at relatively low voltage which is excessively higher than that required for the liquid crystal device. What is worse, the electric optical crystal cannot be used to form a precise array structure. Since the electric optical crystal requires a polarizing plate, there arises a problem of absorption of light due to the provided polarizing plate.